109 SummaryIn three field experiments carried out during 1989-91, a permanent sward of pure white clover (Trifolium repens) was established to provide a source of N for winter or spring wheat crops (Triticum uulgare) directly drilled into the legume. Spring-sown wheat failed to compete with the clover, but wheat sown in the autumn established successfully. N fertiliser was applied to all three experiments at rates of 0,50 and 100 kg N ha-' and measurements of grain and whole-crop silage yields were made. Yields were low for all treatments, probably because of the dry conditions prevailing and the low soil N status of the site used. Yield responses to fertiliser were significant, despite the contribution to plant nutrition that the clover was intended to make. A key feature of the work was that the clover survived successive cereal crops and could be grazed and used as an understorey for later crops. Further, response to fertiliser N diminished with a successive crop implying a build-up of available soil N, which measurements confirmed had occurred. Use of the system obviated the need to use pesticides, although reasons for the lack of pest damage were not clear. Key words: Wheat, white clover, low input, understorey, soil N residues clover. Journal British Grassland Society 31:29-35. Heard A J. 1965. The effect of the nitrogen content of residues from leys on amounts of available soil nitrogen and on yields of wheat. Journal of Agricultural Science, Cambridge 64329-334. Hommay G, Briard B. 1988. Apport du pregeage dans le suivi des peuplements de limacos en grande culture. Haliotes 1855-74. Jarvis S C, Barraclough D. 1991. Variation in mineral nitrogen under grazed grassland swards. Plant and Soil Science 138:177-188. Jones L. 1992. Preliminary trials using a white clover (Trifolium repens L.) understorey to supply the nitrogen requirements of a cereal crop. Grass and Forage Scierzce 47:36&374. Jones L, Clements R 0. 1991. Cereals in clover. Crops, 24 August 1991, pp. 16-17. Ladd J N, Amato M. 1986. The fate of nitrogen from legume and fertiliser sources in soils successively Rhodes I. 1991. Progress in white clover breeding. REUR Technical Series, pp. 1-9. Rome: FAO. Watson E R. 1963. The influence of subterranean clover pastures on soil fertility, 1. Short term effects. Williams E D, Hayes M J. 1991. Growing spring cereals in a white clover (Trifolium repens) crop.

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Whenever supplemental irrigation was applied to sweet potatoes, root quality was reduced. Dry matter content, color of both fresh and processed sweet potatoes, firmness of the canned product, and percent protein decreased as the moisture content of the soil increased. Moisture content had little or no influence on fiber content or cortex thickness of fleshy roots. Nitrogen levels also influenced quality factors. As N levels increased there was a reduction in flesh color and increases in protein content and firmness of the canned potatoes. Nitrogen had little effect on fiber content, dry matter content or cortex thickness. Yearly variations occurred for dry matter content, fiber content, firmness of the canned product and flesh color.

A method of growing cereals with no input of N fertilizer was tested. Two experiments are reported in which white clover {Trifolium repens L.) was used to supply the nitrogen requirements of the cereal. Spring barley, spring oats and winter barley were direct drilled into an established clover crop and grown with the legume as an understorey. The yield of whole crop cereal, for ensilage, and yield of cereal grain were similar to those grown by traditional methods. In the first experiment, clover survived adequately to be used again, but in the second experiment it was killed by a very high slug population. A method of slug control by mob stocking with sheep is discussed. Where clover survived, no weed, insect or disease problems were encountered. The technique offers a possible method of reducing the environmental problems arising from the use of nitrogen fertilizers.